Among other common tasks, in order to change the resolution in your VM you have to install guest additions there. I found all kinds of suggestions for ways to do this. The easiest is to make sure you’re in the VM and hit “HOST + D” where HOST varies but is the right CTRL key by default.

So on my Windows 7 host installation, I just clicked into the Ubuntu VM window and pressed right CTRL + D. One-click install, restart the VM, and you’re done.